Acute myeloid leukemia (AML) is a devastating disease which increases in frequency with aging. Interestingly, the 5-year survival rate for patients less than 65 years of age is about 35%, but drops significantly in patients over 65 years of age to 4%. Elderly AML patients over 65 years of age respond to treatments poorer than younger patients partly due to their reduced tolerance to aggressive therapy including radiation and chemotherapy. Therefore, novel targets for therapies with less toxicity and higher efficacy are needed for this group of individuals. Protein kinases are important therapeutic targets that are considered highly druggable due to their conserved ATP-binding pocket that can occupy small molecule inhibitors, in which prototype of a highly efficient ATP-competitive inhibitor, imatinib (Gleevec) has been described. Interestingly, imatinib achieves its high selectivity by binding to a less conserved region next to the ATP-binding pocket. Thus, designing small molecule inhibitors outside the ATP-binding pocket (i.e. allosteric inhibitors) in less conserved regions of a kinase is likely to result in specific and selective inhibition. Gain-of-function mutations of KIT receptor tyrosine kinase have been associated with acute myelogenous leukemia (AML) and myeloproliferative disease (MPD). A single point mutation of aspartic acid to valine (KITD816V) in the activation loop of KIT, found in up to 90% of SM patients and ~ 50% of core binding factor AML patients changes the wild-type KIT receptor to the extent that it results in altered substrate recognition, constitutive tyrosine autophosphorylation, and promiscuous signaling. As there are currently no efficacious therapeutic agents against this mutant, my project is to define novel therapeutic targets that contribute to aberrant signaling downstream from activation loop KIT mutations (KITD816V) that promote transformation of primary hematopoietic stem/progenitor cells (HSC/Ps) in diseases such as AML and SM. Previously, we and others have demonstrated that the regulatory subunit of class IA phosphoinositol-3-kinase (PI3K), p85a, is required for KITD816V-induced transformation. Accordingly, I hypothesized that effectors of the PI3K signaling pathway, in particular p21 activated kinase (PAK1) and its upstream effectors including guanine exchange factors (GEF) and Rho family GTPases contribute to gain-of-function mutant (KITD816V)-mediated transformation. To test this hypothesis further, I propose three specific aims in this application. The aims are designed to address the specific role of PAK1 and its upstream effectors including Rho GTPase family members as well as GEFs in regulating KITD816V induced transformation using genetic, biochemical and pharmacologic approaches. A series of experiments using knock out mouse models, mouse models of myeloproliferative disorder (MPD), dominant negative approaches and novel allosteric inhibitors of PAK1 have been proposed. I expect the results of these studies to enhance our understanding of the mechanism of KITD816V-induced transformation, and potentially provide novel therapeutic targets for oncogenic KIT bearing neoplasms.